Add server rules (#15769)

app/controllers/about_controller.rb

app/controllers/admin/rules_controller.rb

app/controllers/api/v1/instances/rules_controller.rb

app/javascript/styles/mastodon/about.scss

app/models/rule.rb

app/policies/rule_policy.rb

app/presenters/instance_presenter.rb

app/serializers/rest/instance_serializer.rb

app/serializers/rest/rule_serializer.rb

app/views/about/more.html.haml

app/views/admin/rules/_rule.html.haml

app/views/admin/rules/edit.html.haml

app/views/admin/rules/index.html.haml

config/locales/en.yml

config/locales/simple_form.en.yml

config/navigation.rb

config/routes.rb

db/migrate/20210221045109_create_rules.rb

db/schema.rb

spec/fabricators/rule_fabricator.rb